Endogenous bufodienolides having digitalis-like activity were identified in material extracted and purified from human cataractous lenses by chemical ionization mass spectroscopy, together with UV spectroscopy and biological characterization, as 19-norbufalin and 19-norbufalin peptide conjugate [Lichtstein D., et al., Eur. J. Biochem. 216:261–268 (1993); U.S. Pat. No. 5,874,423].
Since 1969 several attempts to synthesize bufodienolides were reported [Soncheimer F., et al., J. Am. Chem. Soc. 91:1228–1230 (1969); Stache U., et al., Tetrahedron Lett., 35:3033–3038 (1969); Pettit G. R., et al., Can. J. Chem., 47:2511 (1969); Pettit G. R., et al., J. Org. Chem., 35:1367–9 (1970)]. In all reports the starting material was a steroid compound in which the functional groups were altered in order to produce the active bufadienolide molecule. The transformation of digoxin to a bufodienolide compound was also reported [Pettit G. R., et al., ibid.]. In the early 1970, it was already established that for a biologically active cardiolide the C/D cis ring junction, the 17β-butenolide moiety and the 3β,14β-dihydroxy are the main prerequisites. The A/B ring junction can be cis and trans without a dramatic change of activity [N. Almirante, et al., Synthetic Letters, 22:1234–1236 (1998)].
The most advanced synthesis of bufalin and its analogs was reported ten years later [Tsay T. Y. R., et al., Heterocycles, 12:1397–1402 (1979); Sen A., et al., J. Chem. Soc. Chem. Comm. 66:1213–1214 (1982); Wiesner K., et al., Helv. Chim. Acta, 66:2632–2641 (1983); Weisner K. A. & Tsai T. I. R., Pure and Appl. Chem., 53:799–810 (1986)]. According to these reports, a 3β-benzyloxy-α,β-unsaturated-17-keto steroid, was reacted with a lithiated ethyleneacetal of 2-furaldehyde and the product was further transformed in the bufodienolide compound in a multistep process.
In search for synthetic compounds with digitalis-like activity, the present inventors have developed a novel process for the synthesis of 3α and 30β-isomers and analogs of 19-norbufalin, which is the subject of the present invention.
The Na+,K+-ATPase (E.C.3.6.1.3) is an integral plasma membrane protein which is responsible for maintaining sodium and potassium ions gradient in all eukaryotic cells. This enzyme has a high-affinity receptor for digitalis steroids and endogenous ligands for these receptors have been postulated, which regulate the Na+,K+-pump activity. Indeed, digitalis-like compounds have been shown to be present in diverse mammalian and amphibian tissues.
Naturally occurring and synthetic digitalis-like compounds may be used as therapeutic agent in the treatment of various pathological conditions in which involvement of endogenous digitalis-like compounds is implied. Such compounds may be used as cardiotonic agents, increasing the intensity of heart muscle contractions, as vasoactive agents, elevating blood pressure. and as natriuretic/diuretic agents, increasing the excretion of sodium into the urine and thus increasing urine volume. In view of the marked involvement of Na+,K+-ATPase in the central nervous system, digitalis like compounds may also be used as neuromodulating agents. Cardiac glycosides, such as ouabain and bufalin, have been reported to induce cell differentiation and cell apoptosis. The synthetic compounds of the invention can also be used for the treatment of various proliferative cell anid malignant diseases.